24 
GEOLOGY OF THE FIRST DISTRICT. 
Marine Alluvial Detritus. 
Beaches, Shoals, Spits,* ^c. 
The destroying action of the sea upon the headlands and cliffs where currents and a heavy 
surf beat against the coast, has been considered. Another effect of the sea, and as important 
to the community as that just discussed, is the formation of marine alluvion. It results from 
the deposition of the materials transported coastwise by tidal and marine currents, and by the 
action of the waves in the direction of the prevailing winds and storms. The winds which 
produce the greatest transport of alluvial matter on the coast of Long Island, (with the ex¬ 
ception of particular parts, where there are local exceptions in consequence of the form of the 
shore, or direction of currents,) are from the northeast, during the heavy northeast storms. 
These storms bring in a heavy swell from the ocean, which, rolling obliquely along the shore, 
aided by the powerful tidal current, sweeps the alluvia along in a westerly direction. The 
northwest winds are nearly as powerful as the northeast, and blow for a much longer period 
in the year, but do not bring in an ocean swell; and the waves which they raise, fall upon the 
shore in a line nearly perpendicular to the trend of the coast; so that their effect is to grind 
the pebbles and gravel to sand, by the action of the surf, rather than to transport them coast¬ 
wise. In this way, outlets of small bays are frequently more or less obstructed by bars, shoals 
and spits, formed by the tidal currents sweeping past their mouths, and depositing the mate¬ 
rials in the eddy formed by the meeting of the currents. If the strongest currents and pre¬ 
vailing winds be coincident in direction, the outlet of the harbor is on the leeward side. 
When rivers and small streams do not project deltas into the sea, the surf frequently throws 
up a bank of shingle or sand, so as to block up their mouths ; and then, a fresh water pond 
or lake is the result. This effect, however, is much modified by the size of the materials, 
forming the beach across their outlet. If the materials be coarse gravel and pebbles, or shingle, 
the water filters through at ebb tide, so as to retain it near the tide level; but if they be fine 
sand, the water accumulates until it overflows the obstacle, or has a sufficient head to excavate 
a channel through the barrier, and escape. 
Almost every bay, inlet and marsh, on the north coast of Long Island, and also on the south 
coast, where they are not protected from the sea by the long sandy islands which have been 
mentioned in the preceding article, have their outlets blocked up entirely by the materials 
deposited, or so nearly as to leave only narrow entrances. Strong currents set along the 
shore; and these, aided by the oblique action of the surf, roll the pebbles and sand up the 
beach, which, on the retiring of the waves, are swept again into the surf, having described a 
semicircular line, and perhaps progressed several feet by the action of a single wave. This 
Spit, in the usual acceptation of the term, when applied to alluvial deposits, means a long point or tongue of land composed 
of sand or shingle, extending out from the shore, and formed of materials swept along by the tide or waves. 
